The Islamic University of Gaza- Civil and Environmental Engineering Department Public Health (EENV-5325)

Transcription

1 The Islamic University of Gaza- Civil and Environmental Engineering Department Public Health (EENV-5325) Lecture 3: Components of the infectious disease process By Husam Al-Najar

2 Components of the infectious disease process- Chain of infection Causative agent Susceptible host Reservoir Portal of entry Mode of transmission Portal of exit Chain of Infection Etiological agent Source/Reservoir Portal of exit Mode of transmission Portal of entry Susceptible host

3 Modes of Transmission Contact transmission Vehicle transmission Vector transmission

4 Epidemic Curves- Classes of Epidemics A. Common Source Outbreaks: Here the cases of disease arise from a single, shared or 'common' source, such as a batch of bad food, industrial pollution or a contaminated water supply. Controlling the source stops the outbreak. 1. Point source: Here, all cases appear to occur within one incubation period, suggesting that cases did not arise from person-to-person spread. The fact that the outbreak was of short duration suggests that it was a single, brief (hence "point") exposure that did not persist over time. Examples include that embarrassing diarrhea saga following the neighbor's summer barbecue, or a respiratory illness in workers following a breakdown of a fume hood.

5 2. Continuous: As with the point source outbreak, a group of people are exposed to a single noxious influence. But here the exposure continues over a longer time (e.g., a contaminated water supply that doesn't get fixed), so the outbreak persists for longer. The relatively abrupt beginning of the outbreak suggests that many people were exposed simultaneously, rather than it spreading via transmission from one case to another. The fact that no cases arise beyond one incubation period following the termination of the exposure also supports this conclusion.

6 3. Intermittent: You may occasionally see this pattern. This seems to be a common source that is not well controlled, so outbreaks recur. Depending on the time-frame, it could be seasonal or weatherrelated, or perhaps due to a common source such as an industrial contaminant being emitted at intervals. The gaps between the outbreaks might initially suggest person-to-person transmission followed by an incubation period, but this is unlikely because in a transmissible disease the successive peaks would become larger and merge together, as in the next examples.

7 B. Person-to-Person Spread. Here the disease spreads via person-to-person contact the classic infectious disease pattern. Controlling the source is no longer sufficient to control the outbreak. 1. Index case with limited spread: Here a single 'index' case (for example, a returning traveller) infects other people, and cases arise after an incubation period. (Perhaps confusingly, you may also hear this called a point source with secondary transmission). The outbreak wanes when the infected people no longer transmit the infection to other susceptible people, perhaps because of successful control measures (isolation or quarantine). The graph suggests this was achieved quite quickly.

8 2. Propagated: This begins like an infection from an index case but then develops into a full-blown epidemic with secondary cases infecting new people who, in turn, serve as sources for yet other cases. This produces successively taller peaks, initially separated by one incubation period, but the peaks tend to merge into waves with increasing numbers of cases in each generation. The epidemic continues until the remaining numbers of susceptible individuals declines or until intervention measures take effect.

9 Thresholds for detecting an outbreak Disease Outbreak Investigation Some epidemic prone diseases exist in the community even without an outbreak. Outbreaks occur when there is a sharp rise in cases (e.g. Malaria) in Africa for example. Some epidemic prone diseases do not exist normally in the community. The occurrence of just one confirmed case is considered an outbreak (e.g. Cholera). Some epidemic prone diseases are rare and highly deadly when they occur. We do not have to wait for confirmation of a case. Just one suspected case is enough to consider an outbreak (e.g. Ebola) Therefore, thresholds differ from disease to disease Cholera One confirmed case Ebola One suspected case Measles A cluster of 5 or more suspected cases OR at least 3 confirmed positive cases in a catchment area of a health facility in a month Malaria A sharp seasonal rise in cases beyond the usual number of cases

10 1. Verify 2. Recognize the magnitude 3. Diagnose the agent 4. Identify the source and mode of transmission 5. Formulate prevention and control measures Objectives of an outbreak investigation An outbreak comes from a change in the way the host, the environment and the agent interact: This interaction needs to be understood to propose recommendations Host Environment Agent

11 Steps in investigating an outbreak Step 1: Initial response and confirmation of outbreak Assemble team and prepare for an initial field visit as soon as possible Choose a working case definition and confirm cases Find cases systematically Confirm whether there is an outbreak by comparing occurrence of cases with thresholds Describe who is affected, when and where? Step 2: Identify and manage cases Establish a district task force and allocate them their responsibilities Use the working case definition to find cases Set up a treatment centre Step 3: Set up immediate control measures Treat cases to interrupt transmission and reduce deaths Consider vaccination, disinfection and protective wear Provide health education to those at risk Communicate clearly to reduce panic

12 Step 4: Address the resource gaps Ensure adequate medical supplies and logistics to handle cases Look for additional resources to address the gaps Contact Ministry of Health and partner agencies Step 5: Determine responsible factors and make a report Analyse available information to establish the risk factors Prepare a report and disseminate it Recommend and implement priority control measures Step 6: Surveillance: Be on your guard Strengthen existing system to be able to find cases actively Learn from this outbreak to respond better in future Put in place measures to prevent other outbreaks in future

13 Establish diagnosis Identify specific agent Describe according to person, place and time Identify source of agent Identify mode of transmission Identify susceptible populations Disease outbreak Investigation

14 Concepts of Prevention and Control Prevention: Actions aimed at eradicating, eliminating or minimizing the impact of disease and disability, or if none of these are feasible, retarding the progress of the disease and disability. The concept of prevention is best defined in the context of levels, traditionally called primary, secondary and tertiary prevention. A fourth level, called primordial prevention, was later added. Successful prevention depends upon: a knowledge of causation, dynamics of transmission, identification of risk factors and risk groups, availability of prophylactic or early detection and treatment measures, an organization for applying these measures to appropriate persons or groups, and continuous evaluation of and development of procedures applied Preventable Causes of Disease BEINGS Biological factors and Behavioral Factors Environmental factors Immunologic factors Nutritional factors Genetic factors Services, Social factors, and Spiritual factors

15 Levels of Prevention 1. Primordial prevention consists of actions and measures that inhibit the emergence of risk factors in the form of environmental, economic, social, and behavioral conditions and cultural patterns of living etc. It is the prevention of the emergence or development of risk factors in countries or population groups in which they have not yet appeared For example, many adult health problems (e.g., obesity, hypertension) have their early origins in childhood, because this is the time when lifestyles are formed (for example, smoking, eating patterns, physical exercise). In primordial prevention, efforts are directed towards discouraging children from adopting harmful lifestyles The main intervention in primordial prevention is through individual and mass education

16 2. Primary prevention can be defined as the action taken prior to the onset of disease, which removes the possibility that the disease will ever occur. It signifies intervention in the pre-pathogenesis phase of a disease or health problem. Primary prevention may be accomplished by measures of Health promotion and specific protection It includes the concept of "positive health", a concept that encourages achievement and maintenance of "an acceptable level of health that will enable every individual to lead a socially and economically productive life". Primary prevention may be accomplished by measures designed to promote general health and well-being, and quality of life of people or by specific protective measures

17 Primary prevention Achieved by Health promotion Health education Environmental modifications Nutritional interventions Life style and behavioral changes Specific protection Immunization and seroprophylaxis chemoprophylaxis Use of specific nutrients or supplementations Protection against occupational hazards Safety of drugs and foods Control of environmental hazards, e.g. air pollution

18 Secondary prevention: It is defined as action which halts the progress of a disease at its incipient stage and prevents complications. The specific interventions are: early diagnosis (e.g. screening tests, and case finding programs.) and adequate treatment. Secondary prevention attempts to arrest the disease process, restore health by seeking out unrecognized disease and treating it before irreversible pathological changes take place, and reverse communicability of infectious diseases. It thus protects others from in the community from acquiring the infection and thus provide at once secondary prevention for the infected ones and primary prevention for their potential contacts. Secondary prevention attempts to arrest the disease process, restore health by seeking out unrecognized disease and treating it before irreversible pathological changes take place, and reverse communicability of infectious diseases. It thus protects others from in the community from acquiring the infection and thus provide at once secondary prevention for the infected ones and primary prevention for their potential contacts.

19 Disability limitation Tertiary prevention: It is used when the disease process has advanced beyond its early stages. It is defined as all the measures available to reduce or limit impairments and disabilities, and to promote the patients adjustment to irremediable conditions. Intervention that should be accomplished in the stage of tertiary prevention are disability limitation, and rehabilitation. disease Impairment is any loss or abnormality of psychological, physiological or anatomical structure or function. impairment Disability is any restriction or lack of ability to perform an activity in the manner or within the range considered normal for the human being. Handicap is termed as a disadvantage for a given individual, resulting from an impairment or disability, that limits or prevents the fulfillment of a role in the community that is normal (depending on age, sex, and social and cultural factors) for that individual. disability handicap Rehabilitation is the combined and coordinated use of medical, social, educational, and vocational measures for training and retraining the individual to the highest possible level of functional ability.